Abstract
Composite pile foundation has been widely used in ground engineering. This composite pile foundation system has complex pile-soil interactions under seismic loading. The calculation of vertical bearing capacity of composite pile foundation is still an unsolved problem if the soil around piles is partially or completely liquefied under seismic loading. We have completed indoor shaking table model tests to measure the vertical bearing capacity in a liquefiable soil foundation under seismic loading. This paper will use a numerical approach to analyze the change of this vertical bearing capacity under seismic loading. Firstly, the Goodman contact element is improved to include the Rayleigh damping. Such an improvement can well describe the reflection and absorption of seismic waves at the interface of soil and piles. Secondly, the Biot’s dynamic consolidation theory incorporated an elastoplastic model is applied to simulate the soil deformation and the generation and accumulation of pore water pressure under seismic loading. Thirdly, after verification with our indoor shaking table test data, this approach is used to investigate the effects of pile spacing on liquefaction resistance of the composite pile foundation in liquefiable soil. The time histories of pore water pressure ratio (PPR′) are calculated for the liquefiable soil and the vertical bearing capacity in partially liquefied soil is calculated and compared with our indoor shaking table test data at the 3D, 3.5D, 4D, 5D and 6D cases (D is the pile diameter). It is found that the pile spacing has some influence on the extent of soil liquefaction between piles. The vertical bearing capacity varies with liquefaction extent of inter-pile soil. The optimization of pile spacing varies with liquefaction extent. These results may provide some reference for the design of composite pile foundation under seismic loading.
Highlights
A composite pile foundation is different from the traditional pile foundation in its bearing capacity
For the composite pile foundation system in liquefiable sand, its vertical bearing capacity linearly decreases with vibration time and pile spacing, the decrease percentage of the ultimate load before relative vibration gradually increases at the same time of vibration
The vertical bearing capacity of a composite pile foundation with a low pile cap is numerically investigated in liquefiable soil system under a horizontal sine wave load
Summary
A composite pile foundation is different from the traditional pile foundation in its bearing capacity. Qi [14] proposed an experimental method to study the interaction between saturated sand and a model pile based on the principle of effective stress Their experimental method can measure the residual pore water pressure of soil after vibration. Xin [17] introduced a new p-y curve based on the Winkler foundation beam theory in the shaking table test He built a numerical model to analyze the dynamic interaction of liquefied site pile-soil bridge structure, and considered the stiffness and excess pore pressure ratio of soil around the pile. Their results demonstrate that liquefaction in the foundation soil yields the significant degradation of the dynamic spring coefficients and increases the associated damping coefficients under seismic excitation conditions These abovementioned investigations all focused on the horizontal bearing capacity of pile liquefied soil under horizontal vibration loading. Pile foundations is further investigated at 0s, 15s, 25s, 35s in the process of vibration with different pile spacing
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